This invention relates to electrochemical converters employing solid oxide electrolytes and methods for making the same. Such devices perform fuel-to-electricity conversions in a fuel cell (electric generator) mode or electricity-to-fuel conversions in an electrolyzer (fuel synthesizer) mode. The converters are capable of high efficiencies, depending only on the relation between the free energy and enthalpy of the electrochemical reaction and are not limited by Carnot-cycle considerations.
The key components in an electrochemical energy converter are a series of electrolyte units onto which electrodes are applied and a similar series of interconnectors disposed between the electrolyte units to provide series electrical connections. Each electrolyte unit is an ionic conductor with low ionic resistance allowing the transport of an ionic species from one electrode-electrolyte interface to the opposite electrode-electrolyte interface under the operation conditions of the converter. Is is known that zirconia stabilized with such compounds as magnesia, calcia or yttria can satisfy these requirements when operating at high temperature (about 1000.degree. C.). This material utilizes oxygen ions to carry electrical current. The electrolyte should not be conductive to electrons which can cause a short-circuit of the converter. On the other hand, the interconnector must be a good electronic conductor. The interaction of the reacting gas, electrode and electrolyte occurs at the electrode-electrolyte interface which requires that the electrodes be sufficiently porous to admit the reacting gas species and to permit exit of product species.
Solid-oxide devices formed in hollow tubular configurations are known. See, for example, U.S. patent 3,460,991 issued to D. W. White, Jr. on Aug. 12, 1969. Work on such tubular solid-oxide electrolytes also was reported in a publication Thin Film Fuel Cell/Battery Power Generating System (Westinghouse R&D Center 1979). The electrolyte, electrodes and interconnector components disclosed in this work were fabricated using electrochemical vapor deposition (EVD) and layer masking techniques. Additionally, a monolithic honeycomb design was disclosed in a publication Advanced Fuel Cell Development (Argonne National Laboratory 1983). The Argonne Laboratory work employed casting and isostatic forming techniques followed by high-temperature fusion of the components to form a converter.
These prior art approaches have demanded materials having equal thermal expansion coefficients to assure mechanical integrity throughout the temperature excursions encountered in fabrication and usage. This integrity requirement over a wide temperature range has imposed severe restrictions on material selections and consequently in the methods of fabrication.
The concept and approach of forming the components as free-standing plates to circumvent the thermal/structural integrity problem was disclosed by the present inventor in U.S. Pat. No. 4,490,445, issued Dec. 25, 1984, herein incorporated by reference.
There exists a need for more compact, lightweight, efficient converters that are easier to manufacture and more economical in use. In particular, an electrochemical converter employing free-standing plates and having a power-to-weight ratio (in terms of kilowatts per kilogram) of 0.2 or greater would satisfy a long felt need in the industry.